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 Originally posted by arivero A definition of "matter" is lacking here.
How about using the definition given by Einstein in [i]The Foundation of the General Theory of Relativity,[i] Annalen der Physik, 49, 1916
 We make the distinction hereafter between "gravitational field" and "matter" in this way, that we denote everything but the gravitational field as "matter." Our use of the word therefore includes not only matter in the ordinary sense, but the electromagnetic field as well.